The present invention relates to the deposition of soluble materials and in particular to the deposition of soluble materials using inkjet technology.
In recent years there has been an increase in the number of products which require, as part of their fabrication process, the deposition of organic or inorganic soluble or dispersible materials such as polymers, dyes, colloid materials and the like on solid surfaces. One example of these products is an organic polymer electroluminescent display device. An organic polymer electroluminescent display device requires the deposition of soluble polymers into predefined patterns on a solid substrate in order to provide the light emitting pixels of the display device. The substrate may, for example, be formed of glass, plastics or of silicon.
In the manufacture of semiconductor display devices, such as light emitting diode (LED) displays, it has been conventional to use photo-lithographic techniques. However, photo-lithographic techniques are relatively complex, time consuming and costly to implement. In addition, photo-lithographic techniques are not readily suitable for use in the fabrication of display devices incorporating soluble organic polymer materials. Concerns relating to the fabrication of the organic polymer pixels have, to some extent, hindered the development of products such as electroluminescent display devices incorporating such materials to act as the light emitting pixel elements. Consequently, it has been proposed to use ink-jet technology to deposit the soluble organic polymers in the fabrication of electroluminescent display devices.
Inkjet technology is, by definition, ideally suited to the deposition of the above soluble or dispersible materials. It is a fast and inexpensive technique to use. In contrast to alternative techniques such as spin coating or vapour deposition, it instantly provides patterning without the need for an etch step in combination with a lithographic technique. However, the deposition of the soluble organic materials onto the solid surface using ink-jet technology differs from the conventional use of the technology, to deposit ink on paper, and a number of difficulties are encountered. In particular, there is a primary requirement in a display device for uniformity of light output and uniformity of electrical characteristics. There are also spatial limitations imposed in device fabrication. As such, there is the non-trivial problem to provide very accurate deposition of the soluble polymers onto the substrate from the inkjet print head. This is particularly so for colour displays as respective polymers providing red, green and blue light emissions are required to be deposited at each pixel of the display.
To assist the deposition of the soluble materials it has been proposed to provide the substrate with a layer which includes a pattern of wall structures defined in a de-wetting material so as to provide an array of wells or elongate trenches, bounded by the wall structures, for receiving the material to be deposited. Such a pre-patterned substrate will be referred to hereinafter as a bank structure. When organic polymers in solution are deposited into the wells, the difference in the wettability of the organic polymer solutions and the bank structure material causes the solution to self align into the wells provided on the substrate surface. However, it is still necessary to deposit the droplets of organic polymer material in substantial alignment with the wells in the bank structure. Even when such a bank structure is used, the deposited organic polymer solution adheres to some extent to the walls of the material defining the wells. This causes the central area of each deposited droplet to have, at best, a thin coating of deposited material, perhaps as low as 10% of the material in comparison to the material deposited at the walls of the bank structure. The deposited polymer material at the centre of the wells acts as the active light emissive material in the display device and if the polymer material is not deposited in accurate alignment with the wells, the amount and therefore the thickness of the active light emissive material can be further reduced. This thinning of the active light emissive material is of serious concern because the current passing through the material in use of the display is increased which reduces the life expectancy and the efficiency of the light emissive devices of the display. This thinning of the deposited polymer material will also vary from pixel to pixel if deposition alignment is not accurately controlled. This gives rise to a variation in the light emission performance of the organic polymer material from pixel to pixel because the LED's constituted by the organic material are current driven devices and, as stated above, the current passing through the deposited polymer material will increase with any decrease in the thickness of the deposited material. This performance variation from pixel to pixel gives rise to non-uniformity in the displayed image, which degrades the quality of the displayed image. This degradation of image quality is in addition to the reduction in operating efficiency and working life expectancy of the LED's of the display. It can be seen therefore that accurate deposition of the polymer materials is essential to provide good image quality and a display device of acceptable efficiency and durability.